Read the following paragraph and cross out TWO irrelevant sentences.
Black holes can be small or big. Scientists think the smallest black holes are as small as just one atom. When a black hole and a star are close together, high-energy light is made. These black holes are very tiny but have the mass of a large mountain. Another kind of black hole is called “stellar.”(星系)Its mass can be up to 20 times more than the mass of the sun. The largest black holes are called “supermassive，” which have masses that are more than 1 million suns together. Black holes formed when the universe began. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.